optimize png images using zopflipng

[PeterDaveHello]

Re-compress the png images losslessly via Google's zopflipng as below:

 logo/pdf2htmlEX-256x256.png                         | Bin 155348 -> 148028 bytes
 logo/pdf2htmlEX-64x64.png                           | Bin 18513 -> 17272 bytes
 share/pdf2htmlEX-64x64.png                          | Bin 3422 -> 3208 bytes
 test/browser_tests/fontfile3_opentype/bg1.png       | Bin 486649 -> 454282 bytes
 test/browser_tests/geneve_1564/bg1.png              | Bin 177878 -> 90776 bytes
 test/browser_tests/invalid_unicode_issue477/bg1.png | Bin 7944 -> 1029 bytes
 test/browser_tests/text_visibility/bg1.png          | Bin 111893 -> 97249 bytes
 test/browser_tests/with_form/bg1.png                | Bin 9334 -> 4087 bytes